Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
impromptu [ɪmˈprɔmptjuː] adj (celebration, party) импровизи́рованный/импровизи́рован (tactics) непла́новый
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: